Small-bore capillary lasers exhibit very high optical gains and saturation intensities in flowing gas CO.sub.2 systems. However, as a result of the linear axial pressure variation in the capillary tubes, these parameters may vary greatly in the axial direction. The gain per unit length decreases strongly as the tube length is increased. Thus establishing a limit in scaling such tubes to longer lengths. The reason for this decreased gain per unit length as the tube length is increased is due to the nonuniform axial pressure distribution occurring in longitudinal flow. In these waveguide capillary lasers a Hagen-Poiseville flow is established and is sustained by a pressure gradient in the direction of flow. The pressure decreases linearly along the tube axis from some high input pressure P.sub.1 to some lower exit pressure P.sub.o. The equation for such flow is given by ##EQU1## where F is the flux or number of gas molecules flowing per unit time through the capillary of length l, n is the gas viscosity, a is the bore radius, k is the Boltzmann constant, and T is the absolute gas temperature. Thus, the gas flow in the tube depends on the tube dimensions a and l as well as the pressure differential and the temperature. For narrow bore tubes, fast flow or long capillaries the axial pressure differential can become large thereby limiting the optimum pressure, for gain, to a relatively short length of the tube axis. For example, in the case of a CO.sub.2 mixture, pressure ratios P.sub.1 /P.sub.0 of about 5 are required to attain optimum flow rates in 1-mm bore tubes of only 10 cm lengths.
It has been found that this pressure anisotropy can be considerably reduced by using a porous-wall BeO capillary tube which allows the gas mixture to be introduced into the discharge volume with greater axial uniformity. See "Porous-Wall BeO Capillary Waveguide Laser" by A. Papayoanou and A. Fujisawa; Appl. Phys. Lett. 26, p. 158, 15 Feb. 1975, and "Theory of Porous Wall Capillary Tubes for Flowing Gas Lasers" by A. Papayoanou and A. Fujisawa, IEEE J. Quant. Elect., QE-11, p. 579, August 1975. The gas enters the control bore through the pores from a high pressure chamber surrounding the tube. If the pumps are used to pull the gas out both ends of the tube then the highest pressure, P.sub.1, occurs in the center of the tube and the lowest pressure, P.sub.0, occurs at both ends of the tube. The pressure differential is reduced by a factor of four in this manner. Half of this is a result of introducing the gas mixture in a distributed manner through the porous walls. However, the thermal conductivity of even dense BeO tube is low and the thermal conductivity of the porous BeO tubes is 5-6 times lower. Therefore it is necessary to strongly cool the tube along its entire length in order to realize the advantages of the reduced pressure anisotropy. This may be accomplished by using cooling rods or a cooling block contoured to the tube's outer radius; but thermal rods or the block, in order to be effective, covers about half or more of the tubes surface which causes the gas mixture to be introduced nonuniformly radially.